Along with the rapid spread of broadband network, achieving high speed, down-sizing, and high function have been important issues regarding optical communication devices. On the background, there is an increasing need for a semiconductor optical device (OEIC device) in which optical functional devices such as a semiconductor laser device, an optical modulator and a photodiode, which have been used as individual devices, are monolithically integrated on one substrate. The OEIC device has a lot of advantages such as down-sizing by forming a plurality of optical functional devices having different or similar functions on one substrate, cost reduction by reducing workload and the number of parts relating to optical coupling among individual devices, achieving high function and multiple function by integrating a plurality of functional devices having different functions, and so forth. In such an OEIC device, electric insulation between optical functional devices is very important since a plurality of optical devices formed in an array and a plurality of optical functional devices having different electric characteristics are simultaneously driven. For an InP substrate used in a semiconductor optical device in which electrodes are arranged on both surfaces of the device, an n-type or p-type conductive substrate or an insulating substrate is used.
Meanwhile, OEIC devices for optical communication suitable for 1.3 μm band and 1.55 μm band, which are low-dispersion and low-optical-loss bands of optical fibers for communication, are, in view of their easiness in manufacture, preferable to be formed on an InP (indium phosphorus) substrate having semi-insulating (SI) property and have electrodes aggregated on its one side. Such an SI—InP substrate using an InP substrate in which Fe (iron) is doped is disclosed in Japanese Patent Application Laid-Open Publication No. 2002-344087 (Patent Document 1). More specifically, Patent Document 1 discloses a structure in which, in the case of stacking a Zn—InP layer (Zn (zinc)-doped InP) as a p-type semiconductor layer on an Fe (iron)-InP substrate, a Ru—InP (Ru (ruthenium)-doped semi-insulating InP) layer is provided on an Fe—InP substrate, then a stacked body having stacked layers of p-type semiconductor layer, an active layer, an n-type semiconductor layer in this order is provided, and a p-electrode which supplies p-type carriers into the p-type semiconductor layer and an n-electrode which supplies n-type carriers to the n-type semiconductor layer are provided to one surface (a surface to stack the semiconductor layer) of the Fe—InP substrate together to prevent the resistivity as an SI—InP substrate from being lowered due to interdiffusion of Fe in the Fe—InP substrate and Zn in the Zn—InP.